Geological Survey of Finland

Bulletin 259

(formerly Bulletin de la Commission Geologique de Finlande)

Genthelvite-hearing greisens in southern Finland

by Ilmari Haapala and Pentti Ojanperä

Geologinen tutkimuslaitos . Otaniemi 1972

Geological Survey of Finland, Bulletin 259

(formerly Bulletin de la Commission Geologique de FinJande)

GE JTHELVITE-BEARI G GREISE S I SOUTHER FI JLA D

BY

I L M A R I H A A P A L A and PE N T T I 0 J A N PER Ä

\'( ' I T H 8 F I GUR F. S .\ K D 4 T .\ B L E S I 1'\ T 11 E T EX T

GEOLOGINEN TUTKIMUSLAITOS OTANIEMI 1972

H aapala, Ilmari and O janperä, Pentti 1972: Genthelvite-bearing greisens in southern Finland. Ge%gicol SIIrvß)' 0/ Fildond, BII//elin 259. 22 pages, 8 figures and 4 tables.

One small greisen-type genthelvite occurrence has been found in the Eurajoki area and three in the Kymi area. The greisen bodies are associated wirh topaz-bearing biotite granites which represent la te-stage vari etes of the Finni,h rapakiv i granites . The mineral association in the genthelvite-bearing assemblages is: whire mica, chlorite, genthe lvite, fluorite, ± quartz, ± relict feldspar, ± topaz, ± cassiterite, ± su lfides, ± apatite. Quartz is usually the most abundant mineral, but it is not necessarily present. Sulfide minera ls (sphalerite, galena, chalcopyrite) are found in association with genthelvite only in the Eurajoki occur­rence. Genthelvitc is partly replaced by white mica and chlorite in all of the occurrences, and in thc Eurajoki occurrence a lso by the su lfides .

A chemical analysis of the genthelvite-bearing greisen from Eurajoki showed that the rock contains enriched amounts of iron, zinc, a lu ­minium, beryllium, Ruorine and sulfur as weil as reduced amounts of sodium and silica com pared with the unaltered granite. Analyses are also presented for muscovite and chlorite (ripidolite) from one of the Kymi occurrences. Chemical analyses of genthelvite from the Eurajoki and two of the K ym i occurrences showed that the minerals are almost identical in composition. Comparison is made with other genthelvitc occurrences and the formation conditions of genthelvite are discussed, It is suggested that there may be significant ore mineralizations (Sn, Be, W etc.) in association with certain late-stage va rietes ofthe rapakivi granites .

ISBN 951-690-009-7

Helsinki 1972. Valtion painatuskeskus

CO TENTS

Page

Introduction .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

The Eurajoki area . ...... .. .. . .. . . ... . . .. . .. . .. .... . .. .. . . ............ 6

The E uraj oki massif . . . . .. .. . . . . .. . . . ...... . ....... . . ... .. ....... .. 6

The greisen bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

The genthelvite occurrence ..... .. . ............................. . ... 8

The Kymi area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11

The K ymi g ranite complex ........ . ...... . . .... . .... . .... . ...... .. . 11

The greisen bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11

The genthelv ite occurrences . . . .. . . . . . . .. . . .. .. . . .. .. .. . . . . . . . . .. .. .. 12

The mineralogy of genthelvite .............. . . .. .. .. ................ . .. 15

The mineral associations with genthelv ite .. ... . . ....... ............ . .. . . 16

Conclusions 19

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

References . . . .. . . . .. .. . .. ... . .............. . ........ .......... . . . .... 21

INTRODUCTION

Beryllium occurrences are usually divided into two maln types: pegmatlt1C and

nonpegmatitic. Commercial deposits have been almost entirely beryl-bearing peg­

matites. Since the Second World War, however, continuous interest has also been

paid to the study of nonpegmatitic occurrences, some of which are mined for ber­

yllium. The recent discoveries of large nonpegmatitic deposits in the U.S.A. have

shown that the world's principal resources of beryllium are in the nonpegmatitic

deposits (e.g., Shawe 1968, Sainsbury 1968). The nonpegm2.titic deposits (dissemi­

nations in acid igneous rocks, skarn deposits, greisen deposits, quartz-rich veins,

fluorite·beryllium deposits, late-stage formations in alkaline rocks etc.) are essentially

of pneumatolytic-hydrothermal or hydrothermal origin (Beus 1966). The most

~~ ,,~Q o

FIG. 1. Location of the Eurajoki and Kymi areas . The rapakiv i granite massifs and relatcd intrusives are hatched.

6 GeoJogicaJ Survey oE Finland, Bulletin 259

important beryllium minerals are bertrandite, beryl, chrysoberyl, minerals of the

helvite group, herderite, phenakite and barylite.

Since 1967, the Geological Survey of Finland has been prospecting for tin,

tungsten and beryllium in the areas of the Laitila and Wiborg (Viipuri) rapakivi

massifs (age group 1700 m.y.) in the southern Finland. Many greisen bodies asso­

ciated with the late biotite granites have been found during these reconnaissance

studies. The greisen bodies are mainly replacement veins and lodes, rarely pipes,

roundish or irregular formations . Some of them contain variable amounts of sulfides,

cassiterite, wolframite, genthelvite (a mineral new to Finland), beryl or bertrandite.

Besides the cassiterite-bearing greisen bodies, those containing genthelvite

(Zn,Fe,Mn)[BeSi04JaS are especially interesting. Their beryllium content may be remarkably high, but the occurrences so far found are small in size.

The present paper is areport of the genthelvite-bearing greisen occurrences found

in the Eurajoki and Kymi areas in southern Finland (Fig. 1). A short preliminary

description is also given of the granite types and greisen formations of these areas .

Of the authors, Pentti Ojanperä was responsible for the chemical analyses and Ilmari

Haapala for the other laboratory studies and the field ,vork. The latter also wrote

the paper and drew the conclusions.

THE EURAJOKI AREA

The Euraj oki massif

In the Eurajoki area a roundish rapakivi granite massif, 8- 9 km in diameter,

intrudes older Precambrian migmatites. This Eurajoki massif is practically outside,

but just in contact with the great Laitila rapakivi massif, of which it forms apart

as a satellitic massif. Horizontal diabase dikes overlie parts of the contacts between

the granite massif and migmatite (Fig. 2) . The general geology of the Eurajoki area

has been described in brief by Laitakari (1928) and Kahma (1951). The Eurajoki massif consists of an even-grained marginal granite (Tarkki granite),

a more or less porphyritic central granite (Väkkärä granite) and porphyry dikes

intcrsecting the Tarkki granite. The contacts between the granite types are sharp.

The Tarkki granite contains quartz, strongly altered plagioclase, orthoclase, biotite

and chloritized hornblende as the main constituents. In the younger Väkkärä granite

the major minerals are quartz, microcline, albite, biotite and topaz.

The greisen bodies

Greisen bodies occur in vanous parts of the Eurajoki massif (Fig. 2). Most of

the greisens are sterile quartz-mica-chlorite rocks, but many of them also contain

ore minerals (Haapala and Ojanperä 1969). Beryllium minerals are beryl, genthelvite

Ilmari Haapala and Pentli Ojanperd: Genthelvite-bearing grei sens, FinJ and 7

and bertrandite. The Tarkki granite itself contains danalite as an accessory mineral (Haapala and Laajoki 1969).

The important greisen bodies and their interesting mineralizations are marked in

Fig. 2. Sulfides, especially sphalerite, are common constituents in the greisen lodes

and veins in the Tarkki granite and, therefore, are not marked on the map.

There are appreciable quantities of beryl in g reisen -bordered veins in a vem

swarm in the S\V part of the g ranite massif. Quartz, cassiterite, wolframite and

679--!

\

";~ . '.

-cr-",oI·,o ' " __ ::':.-.. ~

/~:;"":;; , '~~C,o,; , '. :.- Ä • , . : "'\-7'~' <;:~~~:" 0,\,

'. ~"':"'" .. : '

. "

\ B. G

\

~c:: ,::'

~ _., ....... • ' _s

" "

2km

FIG. 2. Geological map of the Eurajoki area. (1) Migmatite; (2) d iabase; (3) normal rapakivi granites; (4) the Tarkki granite; (5) the Väkkärä granite; (6) porphyry dike; (7) greisen; (8) d rill ho le; (Be) beryl; (BeB) bertrandite; (BeG ) genthelvite; (Sn) cassiterite; (W) wolframite.

8 GeoJogicaJ Survey of FinJand , Bulletin 259

\ + + + + + + + +

+ + + + + + +

+ +

+

+-+

t- + + + +

t- + + + + +

+ + + +

+ + + + + I.H./ 1971 1m

GJl ~2 ~4 ;;;-- 6

FIG. 3. The genrhelvite-bearing greisen body from Eurajoki. (1) Väkkärä granite; (2) mainl y q uarrz -sericite greisen; (3) quarrz-sericite -chlorite greisen ; (4) quarrz-sericite-chlo rite-genrhel ­

"ite greisen; (5) quartz veinlet; (6) conract showing dip.

molybdenite are usually associated with it. Small amounts of beryl are also met

with in a thin veinlet cutting migmatite on the SE side of the massif. Bertrandite

occurs in variable amounts in some small greisen bodies intersected by drill holes,

about t\\'o kilometers to the west of the Eurajoki church.

The genthclvite occurrence

Genthelvite occurs in a greisen body in the Väkkärä granite. In the outcrop

the greisen appears as an approximately 5 m long and 2 m broad irregular lens (Fig . 3).

The form of the greisen body is obviously controlled by intersecting fractures. Its

three-dimensional shape is unknown. Possibly the body is a carbona or a pipe.

A zonal structure can be recognized in the greisen body. There is usually a seam

of light green rock, consisting mainly of quartz and sericite, at the contact against

the Väkkärä granite . The amount of quartz is higher in this rock than in the original

granite. A broader and darker zone follows composed mainly of quartz, sericite and

chlorite. In the central part and along some fractures in the greisen body there is a

dark grey zone with quartz, sericite, chlorite, genthelvite and topaz as the main

Jlmari Haapala and Pentli OjanpcrCi: Genthelvite·bearing greisens, Finland 9

FIG. 4. A zoned tetrahedral genthelvite crysral (medium grey), partly replaced by a serici te-chlorite-quartz mass (dark grey) and sulfides (white). q is quartz.

Polished secrion, magn. 65 x. Photo Erkki Halme.

FIG. 5. Zonal replacement of genthelvite by chlorite. Some marginal zones of a genthelvite tetrahedron are essentially replaced by chlorite (chI), while the central part (Iower right part of the photograph) is almost unreplaced. In the marginal parts of the crystal there are zones and fragments of unreplaced genthelvite (gh) in chlorite matrix. One nicol, magn. 80 x . Photo Erkki

Halme.

2 12846- ,2

10 Geological Survey of Finland, Bulletin 259

minerals. Sericite and chlorite often replace genthel vite along fractures and crys­

tallographic zones (Figs. 4 and 5). Quartz and topaz sometimes occur as euhedral

inclusions in genthelv ite. The guartz content is cl earl y lower than in the original

granite. Small amounts of fluorite and secondary Fe- Ti oxide occur in all zo nes .

Minute cassiterite crystals are encountered occasionally. In the central parts of the

greisen body there are also irregular distributions of spha lerite, galena and chakopyrite.

These accessory sulfides often fill microfractures in the g reisen and are clearly younger

than guartz and genthelvite. Typically sphalerite and the other sulfides occur in

association with genthelvite veining and replacing it along fractures and crystaIJo ·­

graphie zones in the same way as sericite and chl orite (Fig. 4) .

A 15 kg sampIe taken from the genthelvite-bearing zone was crushed and anal ysed

chemically. The chemical composition is presented in Table 1, compared with that

of unaltered Väkkärä g ranite (locality: x = 678 7 .54, Y = 535.00). According to the

TABLE 1

Chemical analyses of the genthelvite-bearing greisen and the Väkkärä granite from Eurajoki. Thc wet-chemical analyses by Pentti Ojanperä, the spectrognphi c ana lyses (BeO and the elements givcn

in parts per million) by Ri nga D anielsson.

\Xeight per cent Pans per million

Greisen The Väkkärä

granite Greisen Tbe Yäkkilrä

granite

Si02 ........ · . 60.5 2 74.57 Be . .. . · . .. · . 1 400 3 ') T iO., ... . ... . .. 0.04 0.03 Al 2Ö 3 ...... . .. 16.74 14.26

B .. . , . . .. . , .. 15 < 10 Sc . . . . .. . .. . · . 26 < 10

BeO · . 0 • • • • ••• 0. 39 0. 00 Ga . .... . .. . · . 90 48 Fe20 3 . . . .... . . 1. 5 4 0 . 55 Zr .... · . . , . . 74 < 10 FeO · . . , . . . . · . 8. 33 0. 43 Mo ... .. ... .. < 10 < 10 Z nO ... . ... . .. 2.1 3 0. 00 Sn . .. . .. . . .. . 77 100 MnO .. . . . .. . .. 0. 57 0.03 V . .. . . . . . 0 " • < 10 < 10 MgO · . . . . . . . . . 0.04 0. 03 Cr ......... .. . < 10 < 10 CaO . . . . . . . . . . . 0.51 0. 60 Cu . .. . .. . . . .. < 10 < 10 Sr0 1 ) . . . .. . .. 0.0 05 0.00 l i . . ... . . ... . < 10 < 10 BaO l ) .. . .. . . o. 0.0 0. 0 Co . . . .... . .. < 10 < 10 Na.O ..... ..... 0. 08 3.72 K 20 ...... .. ... 4. 06 4. 48

Zn . . . . .. . ... . < 100 Pb ... ... . .. .. 110 19

Li20 .... ... . · . 0.22 0.10 Y ............ 120 53 Rb20 1 ) ... . . .. . 0.200 0. 092 Yb . ... . . ... . 14 10 Nb20 5 J) .... ,. 0. 007 0.0 I 0 La . . . . · . . ... < 10 < 10 P20 5 · . . .. . . . .. 0.03 0. 01 CO, " . , .. . , . ,. 0. 00 0. 00 S ... . · . . ... .... 0 .33 not anal. F ..... 1. 7 8 0.90 B . O + 3.10 0 . 51 B;O - 0.04 0.07

100.662 100. 392

- 0 = F2 . . . · . 0.7 5 0.3 8 - 0 = S 0.16 0.0

99.752 100. 0 12

1) X-ray Ruorescence analysis by Väinö Hoffr':n. 2) In many other sampies the beryllium content is Illuch higher.

llmari Haapala and Pentli Ojanperii: Gcnthelvite-bearing greisens, Finland 11

analyses, the genthelvite-bearing greisen is characterized by almosr complete removal

of sodium, considerable removal of silica and substantial additions of iron, alu­

minium, zinc, beryllium, water, fluorine and sulfur. Potassium, aluminium, iron and

water are included essentially in sericite, chlorite and topaz; zinc, beryllium and sulfur mainly in genthelvite.

A partial microprobe analysis made by Or. Jaakko Siivola showed the chlorite

from the genthelvite-bearing zone to be a very iron-rich variety, almost similar in

composition to the ripidolite from Kymi (Table 2). According to semiquantitative

microprobe determinations, the sericite contains 6.6 % FeO, 10.6 % K 2 0 and 0.3 % ZnO. For want of a more complete analysis the nature of sericite remains obscure,

but it is probably iron-rich muscovite or zinnwaldite. The chemical composition of

genthelvite is presented in Tabi:! 3.

THE KY MI AREA

T h e Kym i granite complex

The Kymi granite complex forms a cupola (Fig. 6) on the present erosion level

III the Wiborg (Viipuri) rapakivi massif. It intrudes older granite varieties, mainly

wiborgite and pyterlite (see Simonen 1965 and Vorma 1971). The complex is roughly

oval in outline, about 5 km in length and 3 km in width. The central part of the

complex consists of porphyritic granite and the margins of even-grained granite.

Both varieties are leucocratic topaz-bearing biotite granites, but the topaz content

in the marginal granite is higher and the biotite content lower than in the central

g ranite . Like the Väkkärä granite in Eurajoki, these granites are characterized by

their anomalously high fluorine, rubidium, tin and beryllium contents. At the outer

contact of the complex, against the »normal» rapakivi granite varieties, there is a

tim composed of pegmatite and even-grained granite. This zone is only a few metres

or less in width and corresponds to the marginal pegmatite (Stockscheider) commonly

met with in the contact zones of the »stanniferous» granites. Besides quartz, feldspars

and mica, the pegmatite often contains topaz in abundance and small amounts of

fluorite, arsenopyrite, tourmaline, molybdenite, bastnaesite and monazite.

The g reisen bodies

Greisen and quartz veins and lodes occur in and around the Kymi granite complex

(Fig. 6). Most of these veins and lodes are barren, but some contain one or several

of the following minerals: galena, sphalerite, chalcopyrite, wolframite, arsenopyrite,

cassiterite, genthelvite and bery!. Beryl is found in greisen-bordered veinlets in a vein swarm cutting the normal

rapakivi (wiborgite) on the SE side of the topaz-bearing granite complex (Fig. 6).

The veinlets consist of quartz, fluorite, beryl, arsenopyrite, \volframite, mica and topaz.

12 Geological Survey of Finland , Bulletin 259

6721~ _______ --""'r'---_____ _____ " o o o o

o o C'

o o o o o c o o o ~\

o o o o o , u . __ ~ -x x

o _ - ,, - x x x_ - --.... .... .... _ .... +

x x ~_.- + ..J.. + + +

+ + + ++ ++ -1;/

+ -l_

+ + +

+ + +

+ +

\ : o

o

o

+ + + + -'- +~ + + + -t,:/ 0

+

+ /

T '

G

o

\) '-' '.\

/ /

+ +/;7 0

/y/ 0 0

:~ /o 0 /

o o

o

o o c

" 0 \ !"b, Zn,Cu 0 \

,

E n

o o o o

o o o

o

o o

o o

o o o

o o

o o

o 0 I HAAPA LA 1971

.,Y p 4 O __ -= __ ~~~==~m

FIG. 6. Geological map of the Kymi area. (1) Norm.l rap3kivi granites; (2) evcn-grained granite; (3) porphyritic granite; (4) porphyry dike; (5) greisen and quartz (broken line) veins or lodes; (6) contact showing dip; (Be) beryl; (BeG) genrhel vite; (As) arsenopyrite; (W)

wolframite; (Sn) cassiterite; (Pb) galcna; (Zn) sphalcritc; (Cu) chalcopyrite.

The genthelvite occurrences

Genthelvite occurs as one of the main minerals in a greisen lode cutting wiborgite (occurrence 1 in Fig . 6) . On the basis of its mineralogical composition, the greisen

can be divided to 1) muscovite-chlorite, 2) muscovite-chlorite-genthelvite, 3) quartz ­

muscovite-chlorite ( -biotite) and 4) quartz-muscovite-genthelvite-chlorite types.

Chlorite rep laces muscovite along smal l fractures. Fluorite occu rs in appreciable

amounts in the different g reisen types. Apatite, monazite, zircon, Fe- Ti oxide,

I1mari Haapala and Pentti Ojanperä: Genthelvite-bearing greisens, Finland 13

' + + + + + + + +

+ +

......

+ +. i

Sm I.H. / 1970

4

FIG. 7. The genthelvite-bearing greisen body (occurrence 1) from Kymi. (1) Wiborgite; (2) greisen; (3) genthelvite-bearing greisen; (4) outcrop.

chalcosite and malacite are occasionally met with, apatite together with genthelvite. Tbe quartz-muscovite-chlorite (-biotite) rock which also contains remnants of felds par occurs mainly at the margins of the greisen body. Because the lode is only partly uncov­

ered, the types are not differentiated on the sketch (Pig. 7). The most prominant types

are the muscovite-chlorite and muscovite-chlorite-genthelvite greisens. The rapakivi

texture (see, e.g., Vorma 1971) of the granite is clearly visible as relict in the greisen

body. Genthelvite occurs primarily as irregular disseminations or as small streaks in the greisen. It is also found in quartz veinlets in the central part of the greisen lode. The disseminated genthelvite crystals ha ve often replaced the plagioclase

mantles around the former potash feldspar ovoids. It is common for the genthelvite

and fluorite crystals to be partly replaced by muscovite and chlorite. Muscovite, chlorite and genthelvite were separated from the greisen variety

containing these minerals as the main constituents with Clerici solution and a Franz

isodynamie separator. Any impurities were then removed by hand picking. In the final muscovite fraction practically no impurities were detectable. The chlorite con­

centrate contained about 2 % muscovite as an impurity. The muscovite and chlorite analyses are presented in Table 2. According to the analysis, the muscovite has

notable high iron and tin contents. No cassiterite was detected in the heavy mineral fraction. The chlorite is high in iron and very low in magnesia and corresponds to

the iron-rich variety (aphrosiderite) of ripidolite (Deer, Howie and Zussman 1962).

14 Geological Survey of Finland , Bulletin 259

TA13LE 2 Chemical analyses and physical properries of muscovite and chlorite from the K ymi occurrence (1) (ana lyses by Pentti Ojanperä) and partial microprobe analysis of chlorite from the Eurajoki occur­

rence (ana lys is by J aakko Siivola).

lon ic rarios on ehe basis of \'\'cighr per cent

24(0. O ll . F) 36(0. O H )

SiOz . . . .. . .. .. . 45. 77 23.03 23 . 6 Si . ..... ... . . . 6.276{ 8.00 5. 316[ 8. 00 TiOz ....... . . . 0.56 0 .52 0. 5 A1l4] · . . . . . . . . . 1. 724 2.684

Alz0 3 ........ . 31. 7 7 20.61 17.8 AIl61 · . . . . . . . . . 3.411 2.923 Fe20 3 .... . ... . 2.43 2.86 Ti . .. . . . .. . , . . 0. 058 0.0 90 FeO . .. . .. .. ... 2.30 35. 09 40. 7 Fe3 + .... . .• .. . 0. 25 1 0 . 497 ZnO .... ... ... 0. 30 1. 73 1. 0 FeZ + ......••. . 0. 264 4.1 4 6 . 774 MnO . . . . . . . . . . 0. 50 5.17 3 . 6 Zn . . . . . . . . . . . 0. 030 0. 295 M g O . . . . . . . . . . 0. 24 0.93 0 . 6 Mn . . . . . . . . . . . 0. 058 1. 0 1 9 CaO .......... . 0. 23 0.0 7 0.1 M g .. .. .. . ... . 0. 049 0 . 320 12. 04 SrO ") . . .. . ... . 0. 00 0 .00 Li . .. . .. .. . . .. 0. 0 17 0. 013 BaO ") ........ . 0. 17 0. 00 Ca .. .. ..... . . 0. 034 0. 017 Na?O ....... . . . 0. 21 0. 03 Ba ........... 0.009 0. 000 K/) . ....... , .. 11. 60 0. 26 0.02 Na .... , .. .... 0.0 56 2.13 0.013 Li20 . . , .. . . . . . 0.03 0.05 K . .... . ...... 2.029 0.077 Rb20 1) ....... . 0.21 0.00 Rb .... ... ... . 0.019 0 .000 Cs20 .......... 0.009 0.00 F . . . . . . . . . . . . 0.260 0 .000 Zr02 ") 0.00 0.00 OH .......... 3. 238 24.00 15. 090 36. 00 Sn ") ......... . . 0. 06 0 .00 ° . ........... 20.502 20.910)

. . ....... . 0.01 0 .09 PzO; COz . . . . . . . . . . . 0.00 0.00 The physical properties

Cl . . .. . ..... . .. 0. 00 0.00 ß ............. 1. 603 ± 0.002 1. 667 :::::: 0.002 F . ....... , .... 0.60 0.0 0 y .. . . ..... . . . . 1. 605 ± 0.002 H 20 + 3. 54 9.8 0 2Va · . . . . . . . . . 24 ± 2' 20 :!:: 5° 1-l20 - 0.06 0. 04 Sp.gr. .. . . . . . . 2.89 ± 0. 02 3.22 :::::: 0.02

..... 1

100. 5991 100.28

- 0 = F2 0.25

1 100. 3491

I) X-ray fluorescence analysis by Väinö Hoffren.

1. Muscovite from the genthelvite occurrence (1) from K ymi . 2. Chlo rite from the gcnthelvite occurrencc (1) from Kymi. 3. Chlori te from the genthelvitc occurrcncc from E urajok i.

The ZnO content (1. 7 3 %) is anomalously hig h, but the tin content is lo\\' . Sphalerite

has not been detected in the greisen lode. It appears that practically all the zinc and

tin of the greisen are in silica tes (chlorite , muscovite, genthelvite).

Small amounts of genthelvite are met with on the northwestern side of the granite

complex in a swarm of g reisen vei ns about 5-10 cm in width (occurrence 2). The

associated minerals are quartz, fluorite, sericite and chlorite. Sericite and chlorite

replace genthelvite and fluorite. An analysis of genthelvite from o ne of the veins is presented in Table 3.

There is a third occurrence of genthel v ite in the Kymi area (occurrence 3) on the

northwestern side of the g ranite complex in an approximately 0.5 m wide g rei sen lode.

I1mari Haapala and Pentti Oianperä: Genthelvite.bearing greisens, Finland 15

In this case minor amounts oE genthelvite are associated with quar tz, re lict feldspar,

sericite and chlorite. Cassiterite and fluorite occur in small amounts.

BeEore genthelvite had been Eound in the described occurrences by the first author

(LH.), the mineral had been identified in the Geological laboratory oE the Outo­

kumpu Co. (Yrjö Vuorelainen, oral communication in 1968) from the sulfide-bearing

lode marked with Pb, Zn, Cu in Fig. 6. The authors oE this paper have not Eound

genthelvite in this lode, so it obviously occurs erratically or in very small amounts.

THE MINERALOGY OF GENTHELVITE

In all oE the described occurrences genthelvite occurs as yellow-brown tetrahedral

crystals, about Y2 cm or less in diameter. Small faces oE negative tetrahedron have

been detected in a Eew crystals. Because such Eaces are visible also in thin seetion,

they can not be due to octahedral c1eavage. In all of the occurrences genthelvite is

partly replaced by white mi ca and chlorite (Figs 4 and 5) , and in the Eurajoki occur­

rence also by sulfides.

Especially in the Eurajoki occurrence, the genthelvite crystals are oEten strongly

zoned . The zonality is usually oscillatory and visible under the microscope as differ­

ences in colour and reEractive index. According to microprobe analyses made by

Dr. J aakko Siivola, the marginal zones are enriched in zinc and the central parts

in iron (Fig. 8).

Genthelvite was separated from three occurrences (the Eurajoki occurrence and

occurrences 1 and 2 in the K ymi area) for chemical analyses and mineralogical studies

Fe Ku Zn K"

Flc . 8. Electron probe scanning pictures showing the distribution of iron (FeKa) and zinc (ZnKa) in a zoned genthelvite erystal from Eurajoki. Magn. 180 x. The iron-rieh mineral surrounding

genthelvite is chlorite.

16 Geological Survey of Finland , Bulletin 259

TABLE 3 Chemical analyses and physical propcrties of genthelvite from ehe Kymi occurrenceS (1) and (2)

and from ehe Eurajoki occurrence . Analyses by Pentei Ojanperä.

Si02 .• .. •• . ••• .. •• .. • ..•..•

AI 20 a ..... . . .. ...•. . .. ..•.. BeO . . ...... ..... ....... .. . FeO . . ............. ... .. . . . ZnO ...... . .......... . ... . . MnO ......... . . . .. .. .. . .. . MgO ....... ... . . . . .. . . . .. . CaO . . .... . ...... . . .. ..... . S ...... ....... . . .. ... . . ... .

- O = S ........ ... .. . . .. . .

Kymi (I)

30. 00 0.0 9

12. 83 2.47

44. 79 6.55 0.01 0.04 5.34

102. 12

2.67

99.45

Number of ions on ehe basis of 26 (0, S)

Zn . ... .... .. . . ...... . .. . . . Mn .. . .................... . Fe ...... . .............. . . . Ca ....................... .. Mg .. . .. . . . ...... . . ... .. .. . Be ....... . . . .... . ..... .... . Si .............. . ......... . AI ..................... .. . . ° ....................... .. S ................. ...... . ..

The physical properties

n ....................... .. .

;~ · r;:) . : : : : : : : : : : : : : : : : : : : : :

6. 528 1. 095 0.40 8 8.04 0.00 8 0.003 6.084 5. 92 1 12.03 0.021J

24.025 1. 975

1. 743 ± 0. 003 3.59 ± 0. 02 8.14 ± 0. 01

K\"mi (2)

30.3 0. 02

12.8 3.5

45 .5 5.0 0.04 0.0 3 5.4

102. 59

2.7

99. 89

6.6 1 0.83 0.58 8.04 0.01 0.0 1 6. 05 5.96 12.01 0. 00

24.01 1. 9 9

1. 743 ± 0.003 3. 60 ± 0.02 8 . 13 ± 0.01

Eurajoki

30. 3 0.08

12.8 4.4

44.4 5.0 0.04 0.11 5. 4

102. 53

2.7

99.83

6.441 0.83 0.72 ~ 8.02 0.02 J 0.01

6.04} 5.9 5 12.01 0.02

24.01 1. 9 9

1. 743 ± 0.00 3 3.58 ± 0.02 8.14 ± 0. 0 1

by the conventional heavy liquid , magnetic and hand-picking methods . No foreign

grains were detectable in the final fractions under a binocular microscope, although they probably contained small amounts of fi ne-grained mica and chlorite as impurities.

According to the analyses, these genthelvites are surprisingly similar in composition

(Table 3). The amounts of the genthelvite end member in these mineral sampies

are 81.2, 82.3 and 80.2 mole per cent. Also the physical properties are nearly similar. In surface sampies genthelvite is often weathered to yellow earthy material which

was identified as goethite b y X-ray technique.

THE MINERAL ASSOCIA TIONS WITH GENTHEL VITE

Genthelvite is obvious ly the rarest of the helvite-group minerals. U ntil the study

of Glass and Adams (1953) it had onl y been found from one locality in Colorado. However, in the late fi fti es and sixties a number of n ew genthelvite occurrences

Localiry

EI Paso County, Colorado, USA

.los, northern Nigeria

Cairngorm Mountains, Scot­land

Lovozero, Kola Peninsula, USSR

K"yv, Kola Peninsula, USSR

Rovgora, Kola Peninsula, USSR

The Oslo Region, Norway

lIimaussay, southern Green­land

Treburland, Cornwall Pitkäranta, Karelia, USSR

Unnamcd locality, USSR Unnamed locality, USSR

Unnamcd locality, USSR

Eurajoki, southwcstern Fin­land

Kymi, southern Finland

TAilLE 4

The occurrences and mineral associations of genthelvit"

Occurrcnce

a) unknown b) cavity in pegmatite In the Pikes

granite c) cavity in pegmatite In the Pikes

granite a) albite vein in granite b) microcline pegmatite in granite pegmatite-lined cavities in adamellite

Peak

Peak

feldspar pegmatites in alkalinc intrusion

pegmatite in gneiss, associated with alkaline granite

pegmatite, associated with alkaline granite

a) unknown nepheline-syenite pegmatite h) nepheline-syenite pegmatite

c) nepheline-syenite pegmatite

albite vein in alkaline intrusion

calc-silicate rock near granite calc-silicate rock near rapakivi granite

silicified syenites in granite vein-type tnctasomatires (quartz -t micro­

cline, quartz T a lbite + microcline, mica feldspar, quartz -1 siderophyllite) in

granite complex

greisen bodies in granite complex

greisen body in topaz-bearing rapakivi granite

greisen lodes associated with topaz-bearing rapakivi granite (three occurrences)

Associatcd minerals

quartz, astrophyllite quartz , microcline, zircon

quartz, m icrocl ine, biotite,

albite, l.i-mica, thorite amazonite quartz, feldspar, kaol in

zircon

feldspar, Mn-ilmenite, zircon, sodalite, apatite

quartz, alll:lZonite, albite, biotitc, museo\! ire beryl, garnet, covellitc, gadolinite

blocky amazonitc, quartz, biotite, ilmenite Auo rite, pyrochlore

not gi"en analcimc, zircon, bastnacsitc, n:1trolitc,

mica, pyrophanite, cudidymite analcime, albite, sphalerite, galena, egirine,

catapleite, astrophyllite, pyrophanitc, Illonazitc, fluorite, 111uscovitc

albite, egirine, neptunite, catapleite

gamet, calcite, chlorite, galena, molybdenite Auorite, biotite, chlorite, vesuvianite

yuartz, albite, pyrite, galena, molybdenite the named mineral assoc iations plus musco­

vite, phenacite, willemite, columbite­tantal ite, cassiterite, zircon, thorite, bast ­naesite, magnetite, galena, molybdenite, sphalerite, pyrite

quartz, siderophyll ite, relict m icrocl ine, willem ite, phenacite, gahnite, zircon, magnetite

quartz, sericite, chlorite, [opaz, Auorite, sphalerite, cassiterite, galena

muscovite, chlorite, Auorite, ± yuartz, ± relict feldspar, + apatite, I: cassi­terite

Rcrcrcnccs

Glass cl 0/. 1944 Glass and Adams 1953

Scott 1957

Knorring and Dyson 1959

Morgan 1967

Eskova 1957

Vasilev 1961

I.unts and Saldau 1963

Oftedal and Saeb0 1963

Bollingcr and Pcterscn 1967

Kingsbury 1961 Bulah and Frank-Kalllcnet­

skiy 1961 Gurv ich cl 0/. 1963 G aletskiy 1966

Gurvich cl ul. 1962 Gurvich 1963, Yurk cl 0/.

1971 The present study

Thc prcsent study

18 Geological Survey of Finland , Bulletin 259

were described from different parts of the wodd. The occurrences and mineral

associations are summarized in Table 4. Genthelvite occurs most commonly in late-stage assemblages in alkali ne or

granitic pegmatites, usually together with albite and/or microdine, although it has

occasionally been found in calc-silicate rocks (skarns) or in metasomatized alkaline

or granitic intrusives. Previous to this study, genthelvite from greisenised rock has only been described

from one (?) area in the U.S.S.R. (Galetskiy 1966, Gurvieh cl af. 1962, Gurvieh 1963, Yurk el al. 1971 ). The papers of Gurvieh et al. (1962) and Galetskiy (1966) make it dear that the occurrences in question are located in a large shear zone in a g ranite

complex, which also indudes arfvedsonite and aegirine syenites. The mineral assem­

blages differ appreciably from those described in this paper. Thus, biotite (sidero­

phyllite), willemite, phenacite and gahnite are not found in association with genthel­

vite in the Finnish occurrences, but are typical of the Russian occurrences. If the

Finnish genthelvite assemblages originally contained biotite, it has now been re­

placed by chlorite and white mica. In the greisen-type occurrences described by

Galetskiy (1966), genthelvite had partly been replaced by mica minerals in the same

way as in the Finnish occurrences.

The rarity of genthelvite compared with many other beryllium minerals indicates

that the mineral can be formed only under relatively restricted conditions. The great

chemical similarities among the Finnish genthelvites further suggest that in these

cases the natural conditions were almost identical.

In Finland genthehrite is dosely associated with fluorite and occasionally with

topaz . This suggests, but does not prove, that fluorine played an important role in

the transportation of beryllium. The extraction of fluorine from the mineralizing

fluids through the crystallization of fluorite and topaz, and the leaching of alkalies

from the surrounding rocks into the solution during g reisenization, obviously

contributed to the deposition of genthelvite (see Beus 1966, p. 348- 360). Several authors (e .g., H o lser 1953, Kingsbury 1961, Beus 1966, Cech and Povondra

1969) have suggested that helvite -group minerals rather than beryl and other Be-Al

silicates are formed in environments poor in aluminium . This theory does not appear

to hold true in the case of the Finnish genthelvite occurrences. In the genthelvite­

bearing assemblage from E urajoki and in the muscovite-chlorite-genthelvite greisen

from Kymi at least, the alumina content is hig her than in the original granites.

However, this contradiction may be only apparent. The alumina is induded in

muscovite and chlorite, in the Eurajoki occurrence partly also in topaz. Part of the

white mica and chlorite d early replaces genthelvite. Thus part of the aluminium was

obviously introduced into the genthelvite-bearing assemblages after the formation of genthelvite.

The helvite-group minerals are chemically intermediate between pure silicates

and sulfides. Besides the necessary concentrations of zinc and beryllium, a proper

(not too low, not too hi gh) activity of sulfur is obviously very important for the

Ilmari Haapala and Pentti Oianperä: Genthelvite-bearing greisens , Finland 19

formation of the species genthelvite (cl Pavlova cl al 1966, Beus 1966, p. 359). In

the Russian greisen-type occurrences the association of willemite with genthelvite indicates a low activity of sulfur (Pavlova cl al. 1966). In the Finnish occurrences

sulfides playasubordinate part or are not present. Only in the Eurajoki occurrence

do appreciable amounts of sulfides occur in the genthelvite-bearing assemblage, and even then th~y were formed later than genthelvite.

Because beryl and genthelvite have not been found in the same mineral assem­

blage, their mutual age relations are not dear. It is worth noting, however, that

such high-temperature minerals as wolframite, arsenopyrite and molybdenite are

not found in the genthelv ite-bearing assemblages, but they occur in beryl-bea ring quartz veins.

Finally, it must be emphasized that extensive experimental studies are needed

for the better understanding of the mutual relations of the beryllium minerals. At

present the stability relations of the tens of beryllium minerals are largely uncertain.

CONCLUSIONS

Several greisen bodies containing genthelvite, beryl or b ertrandite were found

In the Eurajoki and Kymi areas during the reconnaissance exploration work starting

in 1967 in the areas of the Laitila and Wiborg rapakivi massifs. Before these studies

no beryllium minerals had been recorded in artides on the rapakiv i granite massifs

of Finland. In Russian Karelia, a beryl-bearing pegmatite (greisen-bordered beryl­

biotite-plagiodase-f1uorite vein) has been described from Uuksu (Kranck 1929) and

helvite- and genthelvite-bearing skarns from Lupikko in Pitkäranta (Trüstedt 1907,

Saksela 1951, Bulah and Frank-Kamenetskiy 1961). These occurrences are located

outside but near the Salmi rapaki vi granite massif, and obviously have a genetic

connection with it.

In the Eurajoki and K ymi areas the beryllium and other ore mineralizations are

dosel y associated with topaz-bearing biotite granites. These granites are further

characterized by anomalously hig h f1uorine, rubidium, tin and beryllium contents

and an anomalously low zirconium content compared with commo n rapakivi granites

and most granites in general. The mineralizations found in the Eurajoki and Kymi areas and elsewhere in

association with the Finnish rapakivi g ranites show that these granite areas should

not be too much belittled b y prospectors. There may be important ore minerali­

zations (Sn, Be, W, etc.) associated wi th certain types of rapakivi granites. Besides

cassiterite and other metallic minerals, special attention should also be paid to the

beryllium mineral; . Disseminated in greisen bodies, veins, skarns or in altered

g ranite, these minerals may easily b e overlooked.

20 Geologieal Survey of Finland, Bulletin 259

ACKNOWLEDGMEl'\TS

During the flelJ work the first author was assisted by Messrs Pas i Lehmuspelto, Pekka Anttila, Antero Karvinen and Martti Kankkunen, all students of geology. The maps and sketches were redrawn by Miss Marjo Kujala and Miss Lea Leiponen, Mr. Erkki Ristimaa assisted in the mineral separa­tions. Dr. Jaakko Siivola made the microprobe determinations, Miss Ringa Danielson and Mr. Väinö Hoffren the trace-e1ement ana lyses. Mr. Pekka Kallio made the X-ray determinations. Mr. Erkki Halme took the photographs. Mr. Boris Saltikoff translated the Russian papers cited. Dr. Atso Vorma and Dr. Marjatta Okko (the editor) read the manuscript critically.

The authors wish to express their gratitude to all these colleagues and co-workers from the Geological Survey of Finland.

Manllscripl received llllle 12, 1972

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ISBN 951.690-009-7